Data mapper and method for flexible mapping of control and data information within a SONET payload

ABSTRACT

A digital information mapper and method for mapping sequences of information characters into a SONET (synchronous optical network) payload in such a manner that there can be an arbitrary mixture of control and/or data values in those sequences. Each information character comprises 9 bits consisting of an 8 bit information byte, being either a control byte or a data byte, and one octet type bit identifying the byte as control or data. A processor applies a mapping algorithm of a mapping module to the input information characters. The mapping algorithm is applied to sequences of eight input information characters, the algorithm being operable to map the information bytes of each sequence of eight information characters to eight of a sequence of nine contiguous SPE octets. The algorithm then maps the octet type bits for the eight mapped information bytes of the character sequence to the ninth octet of the contiguous SPE octets of the SPE octet sequence.

THE FIELD OF THE INVENTION

This invention relates to a data mapper and method for mapping digitalinformation sequences into a SONET (synchronous optical network) payloadin such a manner that there can be an arbitrary mixture of value types,viz. control and data, in those sequences.

BACKGROUND OF THE INVENTION

A SONET optical data signal (i.e., such as STS-48, meaning 48synchronous transport streams) is typically composed of multiple STS-1'swhich are assigned to various resources or clients, with the greater thenumber of STS-1's representing increased bandwidth. An advantageousfeature of SONET framing, which makes it particularly desirable formetro and wide area optical transport networks, is that it provides adeterministic and flexible bandwidth allocation.

Each STS-1 of a SONET frame has a frame format consisting of rows andcolumns of fixed numbers of octet sequences (an octet having 8 bits andbeing alternatively referred to as a byte) and the first few columns ofoctets contain transport overhead information while the remaining octetsform a payload which transports user information. The payload format fora SONET frame is static and, accordingly, the known methods used for themapping of data into a SONET frame are also static in that each octetposition in the SONET frame payload is assigned a predetermined meaningbetween the sender and receiver of the mapped data. Because of thenature of the SONET payload format the user information which isnormally mapped into a SONET frame is limited to structures havingformats in which control and data type information have either fixedpositions in the input and output information sequences or can bedifferentiated on the basis of algorithms previously applied to themsuch as HDLC byte stuffing.

However, many information sequence structures do not have a fixedstructure and instead may contain an arbitrary or varying mixture ofcontrol and data type values. For example, this is true of packetizeddata such as Ethernet frames, IP datagrams, physical layer encodingschemes such as 8b10b as well as other information structures with twoinformation types (i.e., control and data). For these informationstructures it can be difficult to distinguish between octets containingcontrol information and octets containing data information and thismakes it necessary to add checks to the octet sequences to ensure thatthese information types can be distinguished. For example, forpacketized data such as Ethernet transport data, the above-mentionedmapping scheme based on HDLC byte stuffing is often used for delineatingframes (i.e., marking frame boundaries so as to distinguish between twoframes). This byte stuffing technique defines two control octet values,one used as a frame delimiter code, and the other to mark data codeswith the same value as either control code to prevent misinterpretation.When a data code value matches either control code value, the mark datacontrol code is inserted in front of the data code, and the data codevalue is adjusted.

Such methods are problematic, however, because the number of octetsrequired, and therefore bandwidth required, to transport a data/codesequence is then determined by the data content and as many as twice thenumber of data structure octets may be required to carry the datastructure (since, in theory, it could become necessary to mark each datacode). Extending this technique to carry many different control codevalues, as required for transport of line codes like 8b10b would worsenthis problem. The resulting loss of a deterministic bandwidth capabilitythereby requires additional bandwidth to be provisioned across a SONETnetwork to provide a guaranteed quality of service.

There is a need, therefore, for means to enable a flexible mapping ofdata into a SONET frame by which control and data information valuetypes need not be assigned to any fixed position in the frame and alsoneed not be distinguished by adding extra check codes.

SUMMARY OF THE INVENTION

The present invention provides a data mapper and method for mappingdigital information sequences, which may comprise variable, arbitrarymixtures of control and data value types, into a SONET frame.

In accordance with the invention there is provided a digital informationmapper for mapping input information characters into a SONET framesynchronous payload envelope (SPE) wherein the information characterscomprise control and/or data information and each character comprises 9bits consisting of an 8 bit information octet, being either a controloctet or a data octet, and one octet type bit identifying the octet ascontrol or data. The mapper includes a processor and a mapping moduleconfigured for performing a mapping algorithm, wherein the mappingalgorithm is applied by the processor to sequences of eight inputinformation characters. The algorithm is operable to map the informationoctets of each sequence of eight information characters to eight of asequence of nine contiguous SPE octets and to map the octet type bitsfor each mapped sequence of information octets to the ninth octet of thecontiguous SPE octets of the SPE octet sequence.

In accordance with a further aspect of the invention there is provided amethod for mapping input digital information characters into a SONETframe synchronous payload envelope (SPE). The method includes the stepsof mapping the information octets for each sequence of a plurality ofsequences of eight information characters to eight of a sequence of ninecontiguous SPE octets and mapping the octet type bits for each mappedsequence of information octets to the ninth octet of the contiguous SPEoctets of the SPE octet sequence.

Preferably, the octet type bits for a sequence of information charactersare mapped to the ninth octet of the SPE sequence in such a manner thateach octet type bit is mapped to the bit position of the ninth octetwhich corresponds to the octet position, in the SPE sequence, of theinformation octet with which the octet type bit is associated.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings in which likereference numerals refer to like elements throughout and which show, byway of example, a preferred embodiment of the present invention:

FIG. 1 is a functional block diagram of an optical data (i.e., ESCON)client transmitter having multiple (i.e., 12) serial-to-SONET conversionapparatuses which incorporate a SONET mapper in accordance with theinvention;

FIG. 2 is a functional block diagram of an optical data client receiverhaving multiple (i.e., 12) SONET-to-serial conversion apparatuses whichincorporate a SONET demapper for performing the reverse of the mapper ofFIG. 1 in accordance with the invention;

FIGS. 3(a) and (b) are format diagrams, FIG. 3(a) showing the full SONETSTS frame format and FIG. 3(b) showing only the synchronous payloadenvelope format of the SONET STS frame; and,

FIG. 4 is an information sequence format diagram showing a mapping ofeight ESCON characters (each having 9 bits, the first 8 bitsrepresenting a value which is either a control or data value and thevalue of the 9^(th) bit identifying whether the 8-bit value is controlor data type) into a sequence of nine SONET frame octets, in accordancewith the invention;

FIG. 5 is format diagram showing the mapping of octet sequences (as perFIG. 4) into a SONET payload envelope.

DETAILED DESCRIPTION OF THE ILLUSTRATED PREFERRED EMBODIMENT

FIG. 1 of the drawings is a functional block diagram of an optical data(i.e., ESCON) client apparatus 50, showing its operation in a transmitmode from the client to the SONET network. FIG. 2 shows the operation ofthe same apparatus 50 in a receive mode from the SONET network to theclient. The illustrated client apparatus 50 includes multiple (i.e., 12)serial-to-SONET conversion apparatuses 10 which connect to anaggregator/deaggregator 30 and then to an OC48 framer/deframer 40 whichoutputs and receives SONET frames.

In the transmit mode the mapper/demapper 20 performs a mapping algorithmand the aggregator/deaggregator 30 performs an aggregation algorithmwhereas in the receive mode the mapper/demapper 20 performs a demapperalgorithm (being the inverse of the mapper algorithm) and theaggregator/deaggregator 30 performs a deaggregation algorithm (being theinverse of the aggregation algorithm).

Referring to FIG. 1, in the transmit mode a mapper component of themapper/demapper 20 maps the client data into the SONET payload. This isdone by means of a mapping module, applying a mapping algorithm, whichallocates four STS-1's to a particular client (viz. clients, wherei=1-12 in this embodiment) based on the contents of a configurationmemory (which is not addressed herein and is outside the scope of theinvention claimed by this application). The aggregator 30 performs anaggregation of the client STS-1's into a composite STS-48 payload. Theaggregate data stream is then fed into a standard SONET transmit framer40 to insert section/line/path overhead information (data) and create aproper SONET frame. The parallel output from the framer 40 is thenserialized and converted to an optical signal by a serializer/E-Oconverter (not shown) for transmission over an optical fibre 60.

Referring to FIG. 2, in the receive mode the received optical signal isconverted back to an electrical signal and to parallel format by anO-E/deserializer converter (not shown) and then fed into the SONETdeframer 40 where the section/line/path overhead data is extracted andprocessed. The resulting data stream is passed into the deaggregator 30and then the mapper/demapper 20 which perform the inverse functions ofthose components in the transmit mode. Specifically, the deaggregator 30receives the composite STS-48 payload from the SONET deframer and, basedupon the local configuration memory (again, this aspect being beyond thescope of the invention claimed by this application), it then separatesthe composite STS stream into STS-1's according to the particular clientthey belong to (viz. client_(j), where j=1-12 in this embodiment). Thedemapper 20 then takes the STS-1's assigned to a particular client andextracts the client information from it. As shown, the transmit clockgeneration circuitry (FIFO Depth Monitor, D to A and VCXO) is requiredto adapt the fill rate of the transmit FIFO to the ESCON transmit clockrequirements.

A mapping module of the mapper/demapper 20 performs the mappingalgorithm (method) of the present invention and is described in thefollowing with reference to FIGS. 3(a) and (b), 4 and 5.

For purposes of instruction FIG. 3(a) is provided to show the full SONETSTS frame format and FIG. 3(b) is provided to show the synchronouspayload envelope (SPE) format of the SONET STS frame. As shown by FIG.3(a), the full SONET STS frame is composed of octets positionedaccording to a grid of 9 rows by 90 columns. Of the 90 columns, 3 areused for transport overhead, leaving 87 available as the STS envelopecapacity. Thus, of the total number of 810 frame octets, 27 are used fortransport overhead, leaving 783 octets for STS envelope capacity. Withinthe STS envelope capacity is located the Synchronous Payload Envelope(SPE) which can have an arbitrary phase relationship to the STS frame.The start of the SPE is identified by a pointer in the overhead. Asshown by FIG. 3(b), of the 87 columns available in the SPE one is usedfor path overhead and two are fixed stuff, leaving 84 columns comprising756 octets available to carry payload. Therefore, an OC-n frame hasn×756 octets to carry payload (so an OC48 frame has 48×756 octetsavailable for payload).

The information characters input to the mapper/demapper 20 are 9-bitunits since the client input ESCON line code has an 8b10b structurewhich is converted by the converter 15 to 8 bits of information, beingthe first 8 bits of the character, and 1 bit, being the 9^(th) bit ofthe character, identifying the type of that information (i.e., controlor data type). Thus, an information character comprises an 8 bit value(which may be any value from 0-256) and one octet type bit whichidentifies that value type as either control or data. A mappingalgorithm is applied by the mapper/demapper 20 to map each sequence ofeight 9 bit characters input to the mapper to a sequence of nine octetsof the SPE, wherein the first eight octets of the SPE correspond to thevalues of the eight characters and the ninth octet comprises the inputoctet type bit for each of those eight values to identify them ascontrol or data values. The resulting SPE sequence is shown by FIG. 4.

According to the mapping algorithm a first set of 8 character valuescarrying the 8 bit values of the input characters, shown as octets(bytes) labelled “O 1” to “O 8” in FIG. 4, are mapped sequentially tooctets of the SPE. A 9^(th) octet (shown as the octet labelled “OctetType” in FIG. 4) of the SPE carries each of the 8 bits for those mapped8 character values which identify the type of octet (being either acontrol or data type octet). To map a sequence of 8 input characters tooctets of the SPE the 8 information bits of the first character areplaced in the next available SPE octet and then the same is done foreach of the next seven input characters. The 8 octet type identificationbits for the information characters so mapped are mapped to the 9th SPEoctet in the sequence such that the octet type bit for the first mappedcharacter value is placed in the first bit position of that 9th SPEoctet, the octet type bit for the second mapped character value isplaced in the second bit position of that 9th SPE octet, and so on untilthe octet type bit of the eighth mapped character value has been placedin the last bit position of that 9th SPE octet.

This same process is repeated for further input information charactersbut with a gapping over of the SPE overhead octets 100 and fixed stuffoctets 120 until, as shown by FIG. 5, all octets of the SPE are filledsuch that each set of nine contiguous octets carry 8 9-bit inputinformation characters (it is to be noted that the numbers appearingacross the top of the SPE are to be read vertically, i.e., 01, 02, 03,etc. and, from left to right, run from 1-87). Conveniently, in thisembodiment, because 9-bit information characters are mapped into theSONET frame and the SONET frame has 9 rows, an integer number ofsequences of 9 contiguous octets can be placed into a SONET frame (thisinteger number being 84 in the STS-1 example shown by FIG. 5).

The SPE octet sequences mapped accordingly provide a means for mappingdata character structures having octet type bits (identified as the“CTL” bit in FIGS. 1 and 2) that would not conform to the framestructure of the SONET frame if they were assigned to frame octets, asconventionally done, in a linear manner. This flexibility is provided byaccumulating and grouping octet type bits into an Octet Type octet foreach SPE octet sequence (per FIG. 4).

A demapping algorithm module, which is the inverse of the foregoingmapping algorithm module, is applied by the mapper/demapper 20 toextract, from each sequence of 9 octets of the SPE, the 9 bits for eachof the eight data characters carried thereby. According to the demappingalgorithm, for each 9 octet sequence of an SPE the first eight octetsare extracted (these being “O 1” to “O 8” in the sequence of FIG. 4) andtemporarily held. Then the octet type bits making up the 9th octet,being the “Octet Type” octet of the sequence, are allocated to theirrespective extracted information octets to form the eight 9-bitinformation characters.

Although the example shown by FIG. 5 is one STS-1 only the mappingalgorithm applied by the mapper, and the corresponding demappingalgorithm applied by the demapper, are not limited to any SONET framerate. As will be readily understood by one skilled in the art any STS-n(e.g., STS-48) mapping may be applied in the same manner.

Advantageously, the mapper of the present invention establishes adeterministic bandwidth for transport of information (in doing so,though, it consumes {fraction (1/9)}th of the available bandwidth tocarry data type indicators). Also advantageously, the mapper provides alevel of flexibility which enables up to 256 data and 256 control valuesto be carried within the same SPE (i.e., since the value of each 8 bitoctet is from 0-256 and the 9^(th) bit associated therewith identifiesthat value as being either data or control).

Importantly, the mapper of the present invention allows link extensionacross a SONET network at the physical signalling layer therebyeliminating the complexity and associated cost which would commonly berequired to process the data at higher layers in the protocol stackprior to the transmission and then at the reception of the data on aSONET link.

The terms algorithm, module and component herein refer to softwareand/or hardware means of implementation of the methods described hereinwithout limitation to any specific configuration or operating means.

The individual electronic and processing functions utilised in theforegoing described preferred embodiment are, individually, wellunderstood by those skilled in the art. It is to be understood by thereader that other implementations may be devised by skilled persons forsubstitution while still remaining within the scope of the invention.Persons skilled in the field of communication design will be readilyable to apply the present invention to an appropriate implementation fora given application.

It is to be understood that the particular embodiment shown anddescribed herein by way of illustration is not intended to limit thescope of the invention which is defined by the appended claims.

What is claimed is:
 1. A digital information mapper for mapping inputinformation characters into a SONET frame synchronous payload envelope(SPE) wherein said information characters comprise control and/or datainformation and each said character comprises 9 bits consisting of an 8bit information octet, being either a control octet or a data octet, andone octet type bit identifying said octet as control or data, saidmapper comprising: a processor; and a mapping module configured forperforming a mapping algorithm, wherein said mapping algorithm isapplied by said processor to sequences of eight said input informationcharacters, said algorithm being operable to map said information octetsof said sequences of information characters to eight of a sequence ofnine contiguous SPE octets and to map said octet type bits for each saidmapped sequence of information octets to the ninth octet of saidcontiguous SPE octets of said SPE octet sequence.
 2. A mapper accordingto claim 1 wherein said octet bits for a sequence of informationcharacters are mapped to said ninth octet of said SPE sequence in such amanner that each octet type bit is mapped to the bit position of saidninth octet which corresponds to the octet position, in the SPEsequence, of the information octet with which said octet type bit isassociated.
 3. A mapper according to claim 2 wherein said inputinformation characters are mapped to an SPE comprising four STS-1's. 4.A method for mapping input digital information characters into a SONETframe synchronous payload envelope (SPE) whereby said informationcharacters comprise control and/or data information and each saidcharacter comprises 9 bits consisting of an 8 bit information octet,being either a control octet or a data octet, and one octet type bitidentifying said octet as control or data, said method comprising:mapping said information octets for each sequence of a plurality ofsequences of eight said information characters to eight of a sequence ofnine contiguous SPE octets; and, mapping said octet type bits for eachsaid mapped sequence of information octets to the ninth octet of saidcontiguous SPE octets of said SPE octet sequence.
 5. A method accordingto claim 4 whereby said octet type bits for a sequence of informationcharacters are mapped to said ninth octet of said SPE sequence in such amanner that each octet type bit is mapped to the bit position of saidninth octet which corresponds to the octet position, in the SPEsequence, of the information octet with which said octet type bit isassociated.
 6. A method according to claim 5 whereby said inputinformation characters are mapped to an SPE comprising four STS-1's.